1. Field of the invention
The present invention relates to an apparatus and method for ultrasonic non-destructive testing.
2. Description of the Prior Art
The use of ultrasonic signals in the non-destructive testing of materials is known. Thickness measurements may be carried out by sending ultrasonic signals into a test material and measuring their time-of-flight across the sample. Defect monitoring may be performed by sending ultrasonic signals into a test material and observing their reflection from the structure of a defect. Typically, an ultrasonic transducer is placed in direct contact with the object under test. Transmitted ultrasonic signals are then received by the transmitting transducer also acting as a receiving transducer, or a second receiving transducer may be employed. Such procedures are straightforward in non-hostile environments, but significant technical obstacles must be overcome in order to operate such transducers in hostile (e.g. high temperature) environments.
The development of ultrasonic transducers and their ancillary components capable of withstanding high temperatures for extended periods of time is challenging. Most transducer materials are adversely affected by high temperatures and furthermore, resilient buffer amplifiers are required to convert signals for transmission along coaxial cables, which themselves must withstand the environment. Suitable connectors and power supplies must also be provided.
An attractive alternative would be to use an acoustic waveguide made from a material capable of withstanding the hostile environment to transmit the ultrasonic signal into the test object from a transducer and ancillary components located in a non-hostile region. The end of the waveguide would be attached directly to the region of interest of the test sample. The use of an intermediary waveguide, however, is not a trivial task. Ultrasonic inspection typically employs high frequency (>1 MHz) pulsed waveforms, which are not easily transmitted along a long waveguide with high fidelity, due to dispersion, multiple modes and attenuation. Additionally, both the transducers and the test sample must be efficiently coupled to the waveguide to avoid prohibitively high energy losses.
A major problem to be overcome is dispersion and the presence of multiple modes. FIG. 1 of the accompanying drawings shows dispersion curves for a cylindrical rod waveguide. Some spread in the energy of the transmitted signal is unavoidable, so for example a signal generated at a centre frequency of 2 MHz will typically have energy between 1 MHz and 3 MHz. Hence, since the accurate identification and timing of ultrasonic signals coming from the test sample is paramount to the non-destructive testing procedures described above, it is highly desirable to transmit a signal which is largely non-dispersive, i.e. its velocity is almost constant with frequency, and is dominated by a single mode.
Dispersion in a waveguide and the possible modes are largely a function of the product of the frequency of the signal and the smallest dimension of the waveguide. Furthermore, in order to obtain good accuracy for ultrasonic thickness gauging it is generally necessary to operate at above 1 MHz. However, at higher frequency-dimension products more higher order modes may propagate and thus it is necessary to limit the smallest dimension of the waveguide. Accordingly, the use of thin rod waveguides is known in the art. Such devices are not without their own difficulties though, since it is difficult to transfer sufficient energy into the thin rod to produce a strong signal. Also, when a thin waveguide is joined to a larger structure there is a strong surface reflection and relatively little energy enters the structure. Additionally, a thin rod waveguide coupled to the surface of a structure effectively acts as a point source, from which energy spreads spherically, meaning that little energy returns to the receiving waveguide, even from a strong reflector, such as the bottom surface of the structure.
U.S. Pat. No. 5,962,790 (for example--see Refs 1, 2 and 3 and also Ref 4) discloses a system using thin wire to minimize dispersion and overcoming some of the problems of a single thin wire by employing a bundle of thin wires. Each wire operates at a suitably low frequency-diameter product, yet significantly more energy may be transmitted through the multiple parallel wires in the bundle than through a single wire. Nevertheless, bundles of wires are relatively expensive to produce and become rather inflexible as their diameter increases, limiting the geometries in which they may be deployed. Furthermore, cross-talk between individual wires may complicate the signal analysis and there are practical difficulties associated with either attaching each individual wire to the test structure, or terminating the bundle with a plate which does not introduce dispersion problems. In terms of mode excitation, either extensional modes or a torsional mode may be excited in a single wire. A torsional mode is usually excited by a transducer in contact with the side of the wire, or by an encircling electromagnetic coil. Such techniques are not practical for a bundle of wires, where realistically only extensional modes may be used.
U.S. Pat. No. 6,400,648 (Ref 5) discloses a coiled foil waveguide as an alternative to a bundle of rods. The thickness of the foil is arranged to be much smaller than the smallest wavelength of the propagated signal, satisfying the low frequency-dimension product for non-dispersive transmission. The foil is coiled around an axis parallel to the direction of signal propagation, so if unwrapped would be very long in a direction perpendicular to the direction of signal propagation. However as the diameter of the coil increases, the waveguide becomes rigid and damping due to rubbing between the layers may occur. Like a bundle of wires, a coiled foil is better suited to extensional rather than torsional waves.
U.S. Pat. No. 5,828,274 (Ref 6) discloses a tapered ultrasonic waveguide with an external layer of attenuative cladding. The cladding removes the effects of the waveguide boundaries by damping and limiting surface reflections. This has the effect of removing almost all trailing echoes, however the effects of dispersion are not entirely removed and the signal is slightly delayed, slightly distorted and strongly attenuated. The latter disadvantage limits the length of such a waveguide, which is also rather inflexible. This is an improvement over previous proposals using non-uniform threaded bars as waveguides (see Refs 7 and 8).
U.S. Pat. No. 6,047,602 discloses an ultrasonic waveguide for fluid flow metering which is a rectangular cross sectioned bar with an angled end section. A surface of the angled section reflects energy travelling along the bar into a narrow directed beam to enter the test fluid. The waveguide is designed to maximize the energy transfer across a conduit. This device has significant disadvantages in the field of thickness measurement or defect monitoring, being inflexible and the wave propagation not being optimized for a clean undistorted signal shape, which is of utmost importance for timing measurements in the non-destructive inspection of a sample.
This is a technical problem of providing a practical apparatus for ultrasonic non-destructive testing capable of operating in hostile environments and addressing the above described problems.